Influenza is a major global public health challenge. Although several different influenza vaccines and drugs are available to prevent and treat influenza, in the United States alone each year there are 25-50 million cases of influenza and 30,000 to 40,000 deaths. Controlling seasonal influenza A virus is a challenge because of rapid viral spread; short incubation period; and changing antigenicity of the viral glycoproteins.
Influenza A viruses are negative sense, single-stranded, segmented RNA viruses of the Orthomyxoviridae family. There are several subtypes, labeled according to an H number (for the type of hemagglutinin) and an N number (for the type of neuraminidase). There are 16 different H antigens (H1 to H16) and nine different N antigens (N1 to N9). Each virus subtype has mutated into a variety of strains with differing pathogenic profiles; some are pathogenic to one species but not others, some are pathogenic to multiple species.
The segmentation of the influenza A genome facilitates reassortment among strains, when two or more strains infect the same cell. Reassortment can yield major genetic changes, referred to as antigenic shifts. In contrast, antigenic drift is the accumulation of viral strains with minor genetic changes, mainly amino acid substitutions in the HA and NA proteins. Influenza A nucleic acid replication by the virus-encoded RNA-dependent RNA polymerase complex is relatively error-prone, and these point mutations in the RNA genome are the major source of genetic variation for antigenic drift. Selection favors human influenza A strains with antigenic drift and shift involving the HA and NA proteins because these strains are then able to evade neutralizing antibody from prior infection or vaccination. Antigenic shifts caused three of the major influenza A pandemics in the twentieth century, while antigenic drift accounts for the annual nature of flu epidemics.
Hemagglutinin A is involved in viral attachment to terminal sialic acid residues on host cell glycoproteins and glycolipids. After viral entry into an acidic endosomal compartment of the cell, HA is also involved in fusion with the cell membrane. HA is synthesized as an HA0 precursor that forms noncovalently bound homotrimers on the viral surface, which is cleaved by host proteases to create two subunits associated by a single disulfide bond. The mature HA forms homotrimers having a long fibrous stem comprised of a triple-stranded coiled coil of α-helices derived from the HA2 domains, and a globular head derived from the HA1 domains.
Vaccines that provide for effective protection against influenza infection, and that can address a rapidly evolving virus, are of great medical interest. The present invention addresses this issue.